1. Technical Field
This invention relates generally to an apparatus and method for reducing emissions in a diesel engine, and more particularly to such an apparatus and method which adapts a diesel engine to operate, within preselected speed and load ranges in a convention diesel mode, and in other preselected speed and load ranges in a homogeneous charge compression ignition mode.
2. History of Related Art
Conventional diesel engines tend to emit relatively high concentrations of various oxides of nitrogen (NO.sub.x) and particulate matter (PM), because the diffusion flame, i.e., a long gas flame that radiates uniformly over its length and precipitates free carbon uniformly, results in fuel droplets disbursed in the fuel/air (F/A) charge burning near stoichiometry. A method of combustion identified as Homogeneous Charge Compression Ignition (HCCI), as described in copending U.S. application Ser. No. 08/905,789, filed on Jul. 28, 1997 by the inventor of the present application, has the potential to dramatically reduce NO.sub.x and PM emissions. This is due to the homogeneous mixture of fuel and air being able to provide a uniformly dispersed lean fuel mixture before the start of combustion in each compression stroke of the engine. However, it is difficult for a compression ignition diesel engine to operate in an HCCI mode over a wide speed and load range.
The present invention is directed to overcoming the problem of relatively high emissions in a conventional diesel engine. It is, therefore, desirable to have an diesel engine, and a method of operating a diesel engine in which HCCI combustion is combined with convention diesel combustion so that, when the engine is operating in the speed and load ranges suitable for HCCI combustion, the engine can operate in the homogeneous charge compression ignition mode and produce lower emissions. It is also desirable to have an engine that takes advantage of the low emissions potential of homogeneous charge compression ignition within selected portions of the operating range of a conventional diesel engine by combining the two modes of combustion in one engine.